As shown in FIG. 7(a), in a step of manufacturing a welded pipe, for example, a step of manufacturing a welded pipe by a high-frequency induction welding method, a skelp 1 having a strip shape is processed into a cylindrical body, heated with an induced current generated by supplying a current to a work-coil 2, pressed by welding rollers 3, and then welded at a V-shaped convergent portion (a welding portion) consisting of the skelp edges facing each other.
An impeder, which is placed in a cylindrical body in order to increase the thermal efficiency of welding, includes a case 4 comprising an insulating material and a core 5 comprising a magnetic material, and the impeder is connected to a mandrel 6. The core 5 is cooled with cooling water supplied through the mandrel 6, in order to maintain the performance during welding. The purpose of using the impeder is to increase the thermal efficiency of welding in such a manner that the core 5 is excited with a high-frequency current flowing in the work-coil 2 to prevent an induced current from being supplied to the inside of the cylindrical body to make a welding current supplied through the skelp edges converge on the welding portion.
Thus, cores for impeders need to have the following characteristics: first, high permeability and high saturation magnetic flux density; second, high resistivity, low core loss, and superior workability for forming a shape suitable for cooling; and third, high Curie temperature and small change in magnetic characteristic when the temperature is increased due to heat generated during operation.
As shown in FIG. 7(b), a conventional core includes a ferrite core 5A comprising an oxide-magnetic material prepared by sintering ferrite powder 50. Reference numeral 51 represents a binder.
In high-frequency welding, since a large welding current is supplied and a ferrite core having a small cross-sectional area is used when a pipe having a small diameter is manufactured, the magnetic flux density is readily saturated with respect to the strong magnetic field, thereby decreasing the thermal efficiency of welding. Furthermore, there is a problem in that the saturation flux density of the ferrite core is significantly lowered when the ferrite core generates heat due to the core loss during operation. There is also a problem in that the ferrite core is readily broken because the ferrite core is hard and fragile.
Owing to the above problems, the impeder including the ferrite core has the following disadvantages: low thermal efficiency of pipe welding and short life.
In order to overcome the above disadvantages, an impeder including a core prepared in the following manner has been proposed: a magnetic metal material such as silicon steel having high saturation magnetic flux density is processed into steel sheets 71 having a small thickness and width, as shown in FIG. 8(a), and the steel sheets 71 are bonded with an insulating adhesive agent 72 to form magnetic metal laminations 7, the plurality of magnetic metal laminations 7 are placed in the insulating case 4 to form the core, as shown in FIG. 8(b).
The following techniques for suppressing the heat generation of the impeder are known: a technique (disclosed in Japanese Patent Publication No. H7-14557B) in which magnetic metal wires having an extremely small diameter are tied in a bundle and then stacked, and another technique (disclosed in Japanese Patent Publication No. S61-31959B) in which extremely thin magnetic metal sheets having a thickness of 0.1 mm or less are stacked with insulating materials placed therebetween to form a lamination having a cross-sectional area of 50 mm2 or less.
In the above conventional techniques, the purpose is to obtain a predetermined welding efficiency and to suppress the heat generation simultaneously.
As the demand of steel pipes has been recently increasing, the productivity of the steel pipes must be further raised. Therefore, it is necessary to increase the rate of manufacturing the steel pipes. In the use of the above conventional impeder, a current cannot be sufficiently converged on a welding portion when the pipe-manufacturing rate is raised. In order to achieve a high pipe-manufacturing rate, the capacity of a high frequency power source may be increased. However, such a method raises the manufacturing cost and consumes much energy.
There are many sizes of welded pipes. Since the impeder size must be changed depending on each pipe size, impeders used in a manufacturing process must be replaced when the pipe size is changed. The replaced impeders are put in storage for the next possible use. However, the following situation often arises during storage: the magnetic metal of the core corrodes away due to cooling water being left in the insulating case and therefore the impeders cannot be used again.